The conventional cupola consists essentially of a furnace in the form of a shaft. At the bottom of the shaft is a well portion for collecting the melted metal and for initially receiving a bed charge of coke. Above the well but close thereto are tuyeres for feeding a relatively large volume of air under a relatively high pressure. In the upper portions of the shaft there is provided a charge door.
In conventional operation of the cupola the bed charge is first deposited and extends from the bottom of the well to a point above the tuyeres. The bed charge is ignited while air is injected through the tuyeres at a relatively low rate and maintained until the bed coke is red hot. Then the cupola is fully charged to the charge door sill with alternate layers of flux-containing coke and solid metal.
The air injection is then increased to a predetermined relatively high rate. In consequence the hot coke is further burned and the cupola portion in the immediate vicinity of the tuyeres forms an oxidation zone having a temperature of about 3200.degree. F. The hot gasses now substantially CO.sub.2 rise into the next layer of coke or reducing zone wherein some of the CO.sub.2 is reduced to CO to form an atmosphere having a temperature of about 2400.degree. to 2200.degree. F. When this reducing gas rises to the metal layer, the metal is melted and trickles down into the well to form a pool therein which is tapped and conveyed to a suitable holding vessel outside the well. The flux, basically CaCO.sub.3, combines with the silica and other ash and trickles down to form a slag layer over the molten metal. This slag layer is likewise tapped and periodically removed from the cupola.
The hot gases continue upward through successive layers of coke and metal to preheat these charges and to bring this zone to a temperature of about 1100.degree. F. This exhaust gas which is principally nitrogen from the air, CO.sub.2 and CO is discharged to the atmosphere. In the cupola operation described the sensible heat in the exhaust gas is lost. It has been proposed to divert at least a fraction of these cupola gases to preheat the air fed into the tuyeres using suitable heat exchanger before exhausting it to the atmosphere. It has also been proposed to provide a combustion chamber associated with the heat exchanger and to burn the CO in the cupola gas and hence to extract some of the heat of combustion as well as some of the sensible heat for practical use. It has also been proposed to inject enriching oxygen with the air into the cupola to increase the heating rate.